The telephone lines of telecommunications systems are connected to telephone exchanges or switching centres through the medium of a so-called line interface board which includes such line functions as speech reception and transmission, monitoring the lifting of telephone receivers, and the transmission of telephone ringing signals. Since the line interface boards include electric circuits which are seated in the immediate proximity of the telephone lines, it is necessary to protect the electric circuits against those voltages that can be induced on the lines by lightning strikes, and also against those voltages that can be generated by direct short circuits in the voltage supply power network.
The requirements placed on protective functions against overvoltages will vary from country to country, in dependence on climate, installation rules, etc. For instance, in certain countries power cables and telephone cables are installed in one and the same installation duct. Short circuits can occur between the power lines and the telephone lines in unfortunate circumstances.
The hazardous overvoltages that are liable to enter via the telephone lines can be divided into three different groups, in accordance with the following:
1. Lightning-induced pulses which most often have a duration of less than one millisecond and a peak voltage which is most often less than 2,500 volts.
2. Short circuits between power lines and telephone lines with voltages between 220 volts and 600 volts over a time period of several hours.
3. Internal short circuits in the telecommunication system supply voltage of 50 volts over a time period of some hours.
Gas discharge tubes, thermistors and resistors are examples of the fuses used to protect against such overvoltages.
The requirements specified are different for different types of overvoltage. In the case of overvoltages which fall in the first group, i.e. lightning-induced pulses, the circuits must be capable of withstanding up to about 100 pulses without being damaged. In the case of the overvoltages which fall in the second group, short circuits between power lines and telephone lines, one of the components or circuits present on the line interface board shall function as a fuse and break the current path before there is time for the board or other parts of the telecommunication system to be seriously damaged. When sufficient heat is generated, the component that functions as a fuse will crack, therewith breaking the current path and introducing the desired safety function. In the case of overvoltages that fall in the third group, i.e. internal short circuits in telecommunication system voltage supply circuits the component shall be capable of remaining undamaged over a given limited time period but shall also initiate a safety function should the short circuit prevail over a longer time period. Less heat will be generated in the component in this case, since the short-circuiting current is lower. However, if heat is generated over a longer time period, the component will be heated to an extent which causes the desired safety function to come into effect.
It is known that certain overload cases can be managed with a fuse-functioning resistor. U.S. Pat. No. 3,978,443 teaches an overload protector in the form of a resistor which is connected between substrate-mounted connection plates. When a sufficiently large current flows through the resistor, the resistor and the substrate become heated. The substrate is made of a brittle material, for instance a ceramic material, and will therefore crack at high temperatures. The fuse is constructed so that the substrate will crack, wherewith the current path is broken and the desired safety function is initiated.
It is also known to produce a fuse with the aid of a small metal tab or a small piece of solder-alloy wire which is soldered firmly between two solder terminals in a resistance network. The wire or tab will conveniently be comprised of a low melting solder. As the substrate in the resistance network is heated by the heat emitted from the resistors, the solder will melt and contract onto the solder terminals, therewith breaking the current path and providing the desired safety function.
One problem that arises with this solution is that the solder provided in the fuse will oxidize when heated. When the nature of the load is such that the solder is heated to a relatively high temperature over a relatively long period of time, the solder will oxidize to such an extent as to prevent the desired safety function from coming into effect. This is because the oxide scale or film that surrounds the solder is so strong as to prevent the molten solder from contracting onto the solder terminals, therewith excluding initiation of the desired safety function. In principle, this means that the oxide scale results in the solder retaining its initial form when heated. It is true that the actual solder will melt, but since the solder will not contract the circuit will not be broken.
U.S. Pat. No. 4,973,932 teaches an electric fuse. The fuse includes a short-circuit fuse and a fusible cut-out. The fusible cut-out is partially covered with a thermoplastic material which when overloading occurs penetrates the cavities that appear in the solder in the fuse, wherewith the current path is broken as a result of plastic penetrating the fuse.